The present invention relates to a method of making a building material by activation of latently hydraulic finely ground granulated amorphous basic blast-furnace slag to form a direct acting hydraulic binder, wherein a concrete having great mechanical strength and high chemical resistance is formed. More particularly the present invention relates to a novel activator for a latently hydraulic ground granulated blast-furnace slag and a method for producing said activator.
It is well known that a concrete is formed upon mixing a hydraulic binder, water and aggregates, such as sand and gravel, in varying proportions. One condition is that the hydraulic binder must react chemically with water and, in combination with sand and gravel, produce a concrete of sufficient strength.
Portland cement is generally considered as the best hydraulic binder for mortar and concrete. It forms hardened concrete within a few hours, and the concrete formed obtains its ultimate strength within about one month. The setting and hardening is due mainly to chemical reactions between basic lime and silicic acid.
The hydration of portland cement starts almost instantly upon adding water. The alkali presented in the material activates the subsequent hydration of calcium silicates giving a cement gel, which is responsible for the concrete strength. But there is a main disadvantage in that portland cement develops much more lime hydrate, Ca(OH).sub.2, than the amount consumed by SiO.sub.2. It has been established that portland cement during the hydration process produces about 0.3 kg Ca(OH).sub.2 per 1 kg portland cement. Said excess of Ca(OH).sub.2 was previously considered as an advantage of portland cement, because of the high pH-value of the concrete formed providing a good rust-proof of steel adhered to the concrete. But Ca(OH).sub.2 is an unstable and aggressive compound resulting for example in carbonation shrinkage and subsequent cracking of the concrete. The disadvantage of carbonation of concrete has recently been discussed in an article of M. Maage, Nordisk Betong No 2 (1987). It has been known for many years among scientists that carbon dioxide in the air reacts with the excess of Ca(OH).sub.2 in concrete and gives CaCO.sub.3. Said reaction product, CaCO.sub.3, has a smaller volume than the starting material and the decrease of volume results in shrinkage and cracking of the concrete. Further, carbonation results in a decreased pH-value and the concrete gives less rust-proof of steel adhered to said concrete.
An analysis of portland cement shows about 64% CaO, 20% SiO.sub.2, 2.5% MgO, 6% Al.sub.2 O.sub.3, 3.5% Fe.sub.2 O.sub.3 +FeO, 2% K.sub.2 O+Na.sub.2 O, 1.5% SO.sub.3. The high lime content of portland cement also results in a concrete with limited chemical resistance, which can be seen for example by the fact that concrete pavements are destroyed by road salts, concrete bridges and off shore constructions are destroyed by sea-water and steel adhered to concrete is rusting. A further disadvantage is the fact that portland cement during initial hydration causes a temperature increase, to temperatures over 50.degree. C. Therefore concrete constructions of a thickness over 0.4 m will show deformations and general cracks, because of thermal stress. Different attempts to solve these problems have been proposed, but none of these give a reliable solution. A further disadvantage is that concrete of portland cement is not fire-resistant and cannot withstand temperature over 500.degree. C. The excess of Ca(OH).sub.2 in said concrete decomposes to CaO and H.sub. 2 O at temperatures of 325.degree.-400.degree. C. resulting in shrinkage, increased porosity and decomposition.
It is an object of the present invention to provide a method of making a building material, which material does not show the disadvantages of portland cement mentioned above. The main disadvantage of portland cement is the high calcium content and the fact that not all lime is bound in the hardened concrete. The excess of unstable lime hydrate, which is formed towards the later part of the setting and hardening process, can easily leach out from the concrete by water influence and carbon dioxide in the air, involving a risk of detrimental carbonation.
Measures were taken by the inventor to find a material having a similar composition as portland cement but with a lower calcium content. An analysis of ground granulated blast-furnace slag shows about 30 to 40% CaO, 35 to 40% SiO.sub.2, 7 to 10% MgO, 10 to 20% Al.sub.2 O.sub.3, 0.5 to 2% Fe.sub.2 O.sub.3 +FeO, 1 to 1,5% K.sub.2 O+Na.sub.2 O, 0.5 to 3% SO.sub.3. These figures show that the lime content of slag is only about half the amount of portland cement, but the amounts of SiO.sub.2, Al.sub.2 O.sub.3 and MgO are much higher than in portland cement. These three substances are known to impart to silicates higher chemical and mechanical resistance, i.e. increasing compressive and tensile strength and resistance against chemical influence. From such glass technology it is known that chemical resistant glass such as Pyrex TM has a high content of SiO.sub.2, i.e. more than 80% by weight compared to less than 15% by weight for conventional glass. But substances such as, for example, MgO cannot easily be incorporated in portland cement at low temperatures, it has to be melted into the material, which is difficult because magnesium oxide has a high melting point (2800.degree. C.).
Blast-furnace slag is a nonmetallic product obtained in a molten condition simultaneously with iron in a blast furnace. The slag is considered as a useless residual product and is present in hundreds of millions of tons on an international basis. Granulated blast-furnace slag is the glassy granular material formed when molten blast-furnace slag is rapidly chilled, as by immersion in water or by a combination of cold water and cold air. The slag becomes glassy and amorphous by this process. After the granulated blast-furnace slag is formed, it must be dewatered, dried and ground. Typically, the slag is ground to an air-permeability (Blaine) fineness exceeding that of portland cement, to obtain increased activity at early ages. Typically blast-furnace slag has a fineness of 5000 Blaine (cm.sup.2 /g) and portland cement has a fineness of 3500 Blaine. As with portland cement the rate of reaction increases with the fineness of the material, but for slag in a still higher degree.
The granulated blast-furnace slag is only "latently" hydraulic, i.e. it is not a hydraulic binder, such as portland cement, which directly reacts upon adding water. Therefore the latently hydraulic slag requires admixing with an activator to start the hydration.
Many attempts for activation of blast-furnace slag have been made. The oldest patent is from 1892 (Passow), wherein a mixture of slag and portland cement in an amount of 1:1 is recommended. The lime hydrate, Ca(OH).sub.2, formed in the final stage of hydration of portland cement acts herewith as an activator for the slag. But in many countries this slag cement is hardly used at present, because the initial hydration of slag is slow giving a slow development of strength. Furthermore there is a rather great risk of carbonation shrinkage, because of an excess of lime hydrate in the concrete formed.
Further, alkali salts and sulfates can be mentioned as prior known activators for blast-furnace slag (see H Kuhl, Zement-Chemie Berlin 1951). But these activators must be admixed in rather great amounts into the slag, resulting in a concrete product with a number of deficiencies. For example alkali salts such as NaOH and Na.sub.2 CO.sub.3 must be admixed into the slag in an amount of at least 6% by weight based on the slag. But after activation the setting occurs mostly too rapidly, namely in about 10 to 30 minutes. This rapid setting and hardening makes casting in a building site impossible. Therefore, the use of alkali activated blast-furnace slag is limited to the manufacture of prefabricated components, such as precast concrete units. Further, the addition of caustic NaOH in an amount of at least 6% by weight is inappropriate for the chemical resistance of the product and there is a risk of microcracks. The other known activation method disclosing the use of sulfates as an activator for blast-furnace slag requires admixing of about 10% by weight sulfate into the slag, resulting in an insufficient initial strength and a risk of swelling and shrinkage. Activation by lime, as for example a 1:1 mixture of slag and portland cement, gives a slow reaction rate and the disadvantages mentioned above.
It is an object of the present invention to provide a novel activator, i.e. a novel activation system for a latently hydraulic finely ground granulated amorphous basic blast-furnace slag, which avoids the deficiencies of prior known activation systems. It is a further object to provide a method of making a building material by activation of latently hydraulic granulated blast-furnace slag to form a direct acting hydraulic binder, wherein a concrete having a low calcium content, great mechanical strength and high chemical resistance is formed.